It's more than just Linda that said the pipe wouldn't work as well. And don't pick on her, that flow chart came entirely from things that came out of my mouth.
I used a Homoet 6p (designed for 70cc+) on my 50cc cylinder for about a month; and while it did work GREAT, I had no power until about 40, then it hit HARD. It's not simply because it's labeled as a "high rpm pipe"; it's a function of the chamber diameter and back pressure a pipe can (can't) provide to a smaller displacement. It'd be nearly the same with that 90cc pipe on a 70cc kit. YOU know that, I know that, most of us here know that. But telling that to a 17 year old kid from NJ that is only riding a moped because he can't ride a motorcycle... He'd come back in a week asking why his pipe "doesn't work" until he's going "really fast". Besides, he just wants the pipe because it'll look cool, hence the million questions about which stinger style to go with. NOW he'll probably just go around bragging to his other motorcycle-wanting friends about how his bike has a "90cc pipe on it broooos! fist pump!"
As well, there were a few of us that tried to answer the aforementioned stupid 17 year old's question and be done with it, but he wouldn't have any of it.